1. Field of the Invention
The present invention relates to vacuum pumps, especially primary vacuum pumps used in the semiconductor industry to lower the pressure in process chambers from atmospheric pressure.
The above vacuum pumps must resist many kinds of attacks, resulting in particular from the fact that the gases pumped out of the process chamber are highly corrosive and impose the use of particular materials for making the body of the vacuum pump.
The temperature of the body of a vacuum pump must preferably be controlled, on the one hand to prevent binding due to thermal expansions, and on the other hand to maintain the pumped gases at appropriate temperatures to prevent them being converted into the form of solid deposits in the pump.
Also, vacuum pumps operate in a controlled environment of the white room type, in which cooling systems cannot employ a flow of air. It follows that the temperature of the pump body must be controlled by circulating a cooling liquid.
The temperature of a vacuum pump body in operation is relatively high, and there is therefore a high risk of corrosion of the walls of the cooling circuit of the pump by the cooling liquid, and a risk of deposition of material on the walls of the cooling circuit.
2. Description of the Prior Art
One current solution to the problem of cooling a vacuum pump is to use an anticorrosion coating in the cooling circuit. The anticorrosion coating, for example a coating of polytetrafluoroethylene, is deposited on the stationary parts of the pump body to isolate the cooling liquid, such as deionised water, from the materials of the body of the vacuum pump. A disadvantage is that an anticorrosion coating of this kind is generally made from a thermally insulative material, which reduces the thermal exchange and the cooling capacity of the circuit. Another disadvantage is the risk of local defects in the continuity of any coating obtained by depositing a material. As a result of this reliability and efficiency are not satisfactory.
It has also been proposed to attach to the body of the vacuum pump metal parts which at the same time conduct the cooling liquid, provide resistance to corrosion, and convey heat between the vacuum pump body and the cooling liquid. This solution increases the overall size of the pump, and increases the cost of manufacture of the vacuum pump.
The document JP 05 118 288 describes a vacuum pump in which each bearing and gear housing is cooled by a fluid flowing through a stainless steel tube cast into the housing. This solution has many disadvantages. First of all, the casting operation is always difficult because of the difficulty of placing and retaining the tube in the casting mould. Next, the embedded tube is so intimately bonded to the housing that it is practically impossible to change it afterwards. And most importantly, the metal constituting the housing body is generally a metal with a high melting point, and is therefore cast at a temperature at which the tube looses its anticorrosion properties. As a result, corrosion protection is insufficient.
Also known in the art is the expansion by rolling technique, which is mainly used to make a rigid mechanical connection between the end of a tube and a support plate. To achieve this, an expander tool is inserted into the interior of the end section of tube, and the tube is deformed radially beyond its elastic limit to take up the clearance between its outside diameter and a bore in the plate; the expander tool then applies further radial deformation and expands the tube to make the mechanical connection between the tube and the support plate.
The invention aims to design a new vacuum pump cooling system structure, and a method for making it, providing at the same time good thermal conduction between the cooling liquid and the vacuum pump body, further reducing the risks of corrosion by the cooling liquid, and reducing the cost of manufacture. The invention must enable the use of appropriate materials suitable at the same time for the cooling liquid and for the materials constituting the vacuum pump body, and prevent any overheating likely to degrade the anticorrosion properties of the materials employed.
The invention also aims to facilitate subsequent changing of the anticorrosion materials, if necessary, depending on the conditions of use.
The idea which is at the basis of the present invention consists of using the expansion technique by applying it in a novel manner to making an anticorrosion wall in a vacuum pump body cooling circuit.
To that end, the invention provides a vacuum pump cooling system including a cooling circuit in which a cooling liquid flows in the vacuum pump body and is isolated from the material constituting the vacuum pump body by at least one layer of anticorrosion material preventing corrosion of the vacuum pump body by the cooling liquid; said layer of anticorrosion material includes at least one tube of a material that is resistant to corrosion and a good thermal conductor and is expanded into a corresponding housing in the vacuum pump body.
In routine applications, the vacuum pump body is made of cast iron. The tube can then advantageously be made of stainless steel. The cooling liquid can be deionised water. The expansion is effected cold, and avoids degrading the stainless steel when the tube is assembled into the vacuum pump body, degradation which could happen if the tube were fitted by a heat treatment or by casting the cast iron around it.
As an alternative to this, with a cast iron vacuum pump body, tubes of a metal or light alloy such as copper, monel or inconel can be used. The expansion technique enables the use of these materials without degrading their anticorrosion properties.
In any event, a tube of a material that is not corroded by the cooling liquid is chosen. The expansion technique allows a wide choice of materials, and therefore effective matching to different cooling liquids and to different materials constituting the vacuum pump body.
In the case of a tube made of stainless steel, which is not very malleable, the interface between the tube and the vacuum pump body can advantageously be smooth.
In contrast, if the tube is made of a material that is more ductile, such as copper, the interface between the tube and the vacuum pump body can be rough. For example, a bore can be provided in the vacuum pump body whose surface state prior to the expansion step is rough.
The material forming the tube is preferably expanded throughout the length of the housing in the pump body that receives it.
The invention also provides a method of producing the above kind of vacuum pump cooling system, including a step of radially expanding the tube beyond its elastic limit in the corresponding housing of the vacuum pump body, by applying an appropriate pressure to the inside of the tube.
The appropriate pressure can be applied by an expandable sleeve fed with a hydraulic fluid under pressure.
Alternatively, the appropriate pressure can be applied by a mechanical expander tool.
Other objects, features and advantages of the present invention will emerge from the following description of particular embodiments, given with reference to the accompanying drawings.